The autumn equinox, otherwise known as the first day of fall, will officially arrive today (September 22) at 4:02pm on the East Coast.
The equinox occurs because of the tilt of the Earth in relation to the sun. This is what causes the seasons.
The Earth's tilt is 23.5 degrees relative to the plane of its orbit and means that, although one revolution of the planet takes 24 hours - it's different depending on the time of year.
During the summer time, the Northern hemisphere is tilted towards the sun. So we get longer days as more light falls on this part of the surface.
In the winter time it's the Southern hemisphere that gets the majority of the light.
Find out how to celebrate the autumnal equinox at Bustle.com.